The invention relates to a film capacitor and a film for a film capacitor. The film capacitor is constructed from at least one carrier film with at least one conductor layer, the at least one conductor layer being divided into sectors and the sectors being mutually arranged in such a way that the film capacitor has an internal series circuit of elementary capacitors.
Film capacitors exist for a wide capacitance range. In general, the dielectric in such film capacitors comprises a plastic film. The electrodes comprise conductive metal areas. These metal areas are either thin conductor films or conductor layers vapor-deposited onto the plastic film. If the conductor layers are thin enoughxe2x80x94they are often Al or Zn alloys applied in a vacuum with a thickness in the range of 10-20 nmxe2x80x94, there is a self-healing effect in the event of electrical breakdowns, i.e. the metal layer evaporates in the vicinity of the breakdown location. In the event of an electrical breakdown, the metal layer evaporates in the vicinity of the discharge or at a point provided for this in proximity thereto. After a breakdown, the capacitor can still be used, although with a slightly reduced capacitance.
The prior art discloses films for film capacitors which have segmented metallizations. The individual segments of the metal coating have a low sheet resistance and are isolated from one another by trench-like cutouts. The segments are connected by conductor bridges with a small cross section. These conductor bridges serve as protection devices which, in the event of an electrical breakdown, isolate the affected segment from the remaining segments. If a local breakdown occurs, the power liberated at the breakdown location is limited by the protection devices, as a result of which relatively great damage can be avoided.
DE-A 198 56 457 describes a film having conductor layers with a current path structure formed thereon. The conductor layers themselves have a high sheet resistance for the purpose of reducing breakdown-governed capacitance losses. The current paths run in the conductor layer and have a lower resistance than the conductor layer for the purpose of reducing the total sheet resistance. The current path structure has a main current path in the film direction and auxiliary current paths branching therefrom.
GB 2 276 765 discloses multilayers of capacitor films for internal series circuits. The electrodes exhibit segmentation and have zones with high electrical resistance, where the capacitance is produced, and zones with low electrical resistance at the locations of the connecting areas.
U.S. Pat. No. 5,757,607 shows unsegmented multilayer electrodes with an internal series circuit and a profile in the metallization.
Existing electrodes for high-voltage capacitors with an internal series circuit require a relatively high conductivity and conductor bridges between the segments or profiled electrodes. Such a capacitor design has disadvantages with regard to the self-healing properties. Owing to the high electrode conductivity, during the self-healing process a relatively high energy is liberated and a relatively large area is made unusable. If, however, on the other hand, the electrode conductivity is reduced, this results in a rise in the losses.
In the case of solutions with segments which are connected by conductor bridgesxe2x80x94protection devicesxe2x80x94, an additional problem results in combination with the internal series circuit. After burning through the protection device, individual segments can be completely disconnected and no longer have a defined electrical potential. This can lead to undesirable charging effects. Therefore, segmented electrodes have sooner not been taken into consideration heretofore for high-voltage capacitors. Thus, there is an unchanged high demand for a self-healing capacitor with an internal series circuit.
Thus, it is an object of the invention to provide a capacitor for high voltages which overcomes disadvantages of existing capacitors and which, in particular, is self-healing in the event of a breakdown without an excessively high energy being liberated in the process. Moreover, the capacitor is also intended to operate in low-loss fashion in alternating-current applications.
An exemplary embodiment of a capacitor according to the invention has at least one carrier film as a dielectric and has an internal series circuit. The latter is brought about by the conductor layer applied on the dielectric or the conductor layers being divided into sectors which are mutually arranged and, if appropriate, connected to one another in such a way that a plurality of series-connected elementary capacitors are formed. It is distinguished essentially by the fact that a current path structure is formed on the sectors, the conductor layer having a high sheet resistance for the purpose of reducing breakdown-governed capacitance losses and the current paths of the current path structure having a significantly reduced sheet resistance for the purpose of reducing the total sheet resistance. The sectors may be formed in such a way as to produce an internal series circuit of two, three, four, five, six or more elementary capacitors.
In the event of a breakdown, a conductor layer sheet resistance that is as large as possible limits the region where the conductor layer evaporates. As a result, the capacitance of the capacitor is correspondingly reduced only a little; moreover, less current flows and the energy loss during a breakdown is low. This results in a lengthened capacitor service life. The fact that the total sheet resistance is reduced by current paths with a resistance that is as small as possible means that there is also a reduction of the ohmic losses during a charging or discharging process in normal operation. A capacitor according to the invention with an internal series circuit for high-voltage applications thus makes it possible that, in the event of breakdowns, the capacitance is reduced only a little and, nevertheless, only small losses arising in the form of heat in the capacitor have to be accepted. Moreover, it can manage without conductor bridges serving as protection devices. Therefore, it is also not readily possible for the situation to arise wherein a disconnected segment no longer has a defined potential.
In accordance with one embodiment, the sectors of the conductor layer each have a main current path and auxiliary current paths. The sectors run in strip form, for example, the main current paths then running essentially along the strip longitudinal axes.
For the purpose of additionally reducing capacitance losses in the event of breakdowns, the individual sectors of the capacitor may, for their part, comprise segments that are electrically connected to one another. Auxiliary current paths may run in the individual segments, the segments being interconnected by main current paths. As an alternative to this, however, the segments may also be connected by conductor bridges in a manner known per se.
The surface resistance of the conductor layer is preferably at least 5xcexa9, particularly preferably at least 20xcexa9. The sheet resistance of the current paths is significantly lower, preferably by at least a factor of 20 and, for example, by at least a factor of 50. Correspondingly, the current paths may be formed in such a way that they only cover a small part of the area of the conductor layer, for example 10% or less.
The current paths may be formed by local thickened portions of the conductor layer, as a result of which the current path structure has a contour elevated over the electrically conductive structure. As an alternative to this, it is also possible to provide a framework of metal strips made of a different material on the conductor layer. In principle, the conductor layers could even have interruptions at the locations of the current paths, said interruptions being bridged by the metal strips forming the current paths.
The film capacitor is wound, for example. To that end, in accordance with a first example, it is possible to use two carrier films which are coated on one side and whose conductor layer sectors forming the electrodes are arranged offset with respect to one another, thereby automatically producing a series circuit. As an alternative to this, it is also possible to use a carrier film provided with a conductor layer on both sides together with an uncoated carrier film. The sectors of the conductor layers on the two sides of the carrier film are then likewise arranged offset with respect to one another.